Numerical computation of radar echoes measured by MARSIS during phobos flybys

The Mars Advanced Radar for Subsurface and Ionosphere Sounding, ldquoMARSISrdquo, on board MarsExpress is the first and so far only space borne radar that observed the Martian moon Phobos. Radar echoes were measured for different flyby trajectories. The primary aim of the low frequency sounding of the crust of Phobos is to prove the feasibility of deep sounding. In this paper we present a numerical method that allows precise computation of radar echoes backscattered from the surface of large objects. The software is based on a combination of a Physical Optics calculation of surface scattering of the radar target, and a Method of Moments approach to calculate the radiation pattern of the whole space borne radar system, whereby the calculation of the frequency dependent radiation pattern takes into account all relevant gain variations and coupling effects aboard the space craft. This paper explains the simulation techniques and presents a comparison of simulation results for different orbits, and an interpretation of the backscattered signals.